the skirt length is already being added to the "SkirtLength" column in the ecart object, there is no need to capture it in another session variable.
On the calculation tab of the eCart object, create a new calculation:
(([SkirtLength] > 6)?12:0) * [Quantity]
then on the charges tab, create a new charge:
Subtotal For Any Column:
If the subtotal of column 'SkirtCalc' is '>' a value of '0'
based on Column subtotal:
subtotal of column 'SkirtCalc' 'plus' '0'